Spoke length calculator
Exact 3D-trig math, left and right computed separately, asymmetric rims handled, with a live hub view drawn to scale. Pick a hub and rim or enter the values manually.
New to the terms? Glossary → · Build a wheel from start to finish → · Build wizard →
Loading the hub/rim database…
Higher-tension side = 100%; the other is its share.
Open hub and rim database, validated against manufacturer specifications. Verify ERD and offset with the exact nipples you'll build with: that is where most length errors come from. Then cross-check against a second calculator and plan on a test build.
How the math works
Spoke length is the distance from the flange hole to the nipple seat, solved with exact 3D trigonometry — no lookup tables, no approximations. W is centre-to-flange, D is flange diameter, ERD is the effective rim diameter, X is the cross count, N the spoke count and s the flange hole diameter. Each side is solved with its own W and D, and the calculator shows every intermediate value so you can check the numbers.
L = √( W² + (D/2)² + (ERD/2)² − 2 · (D/2) · (ERD/2) · cos θ ) − s/2 θ = X · 720° / N
FAQ
- Why do the left and right sides get different lengths?
- Because the flanges sit at different distances from the wheel's centreline (dish). On a rear or disc wheel the drive or rotor side has less centre-to-flange, so its spokes are shorter and tighter. A rim-brake front is symmetric, so there both sides come out the same. Order the two lengths the calculator gives you; never average them into one.
- What is ERD and why does it matter so much?
- ERD (Effective Rim Diameter) is the diameter of the circle the spoke tips reach inside the nipples, and it is the single largest source of length errors: 1 mm of ERD moves each side by about 0.5 mm. It depends on your nipples as well as the rim, so measure it yourself. How to measure ERD →
- Should I round the result up or down?
- Down by default: a spoke that sits slightly deep in the nipple beats one that runs out of thread. Spokes are sold in 2 mm steps (1 mm in some ranges), each side is rounded independently, and there is a Nearest option if you prefer the closest stock size.
- Does it handle asymmetric rims and straight-pull hubs?
- Yes. An asymmetric rim's offset spoke bed (OSB) shifts the nipple line to one side and changes each side's length; enter the offset and each side is corrected separately. Straight-pull hubs use the manufacturer's per-side spoke offset instead of a flange hole position — and with no J-bend elbow to absorb slack, a 1 mm measuring error is a 1 mm length error.